High-quality recycling of plastic components requires that the used parts or production residue collected should be as nearly as possible composed of a single type of material, i.e. not comprise foreign plastics. However, sorting and/or separation for this purpose is often difficult and a mixture is therefore often inevitable. Examples of consequences of certain proportions of foreign plastics are impairment of properties of the main plastic, or impairment of processing in the melt due to decomposition reactions.
Polyacetal is often found as contaminant in the plastics. The decomposition reactions can be controlled by matching the processing conditions, such as temperature and residence time, to the main plastic, but this procedure cannot be used for polyacetal present as contaminant. In addition, it is likely that the main plastic, which may contain acidic end groups, or comprise additives or catalyst residues, will react with the polyacetal.
It is particularly during recycling that thermoplastics may comprise undesired admixtures of polyacetal. The polyacetal contamination generally enters the plastics to be recycled when polyacetal components are not isolated, or cannot be isolated, from the actual plastic components to be recycled. Another way in which polyacetal contamination can enter other plastics is through inappropriate operation with inadequate cleaning of the apparatus after production of previous products which comprise polyacetal.
Formaldehyde can form during processing in the melt of thermoplastics which comprise small admixtures of polyacetal. The polyacetal depolymerizes or decomposes in the melt. Even small amounts, e.g. less than 0.5% by weight, of polyacetal in the main plastic can be sufficient to liberate undesirable concentrations of formaldehyde.
It is well known that polyacetals can decompose thermally during processing in the melt, thus liberating formaldehyde.
High processing temperatures above 230xc2x0 C. and long residence times accelerate the degradation (Becker/Braun, Kunststoff-Handbuch, [Plastics Handbook], Volume 3/1, Polycarbonate, Polyacetal, Polyester, Cellulose ester, [Polycarbonates, polyacetals, polyesters, cellulose esters], Carl Hanser Verlag Munich, Vienna, 1992). In addition, polyacetals can react with acidic compounds, additives, catalyst residues, etc., with liberation of formaldehyde.
The Bayer A G publication ATI 0305 xe2x80x9cRecycling und Designxe2x80x9d [Recycling and Design] discloses that admixture of polyacetal in the recycling process should be avoided since it results in inadequate properties of the main plastic. The formation of formaldehyde from admixtures from polyacetal in other plastics is described, taking the example of polyester recycling.
Fleischer et al. in Kunststoffe 9/92, describe the product recycling and feedstock recycling of pure polyacetal.
It was therefore an object to reduce the amount of formaldehyde liberated during the processing, in the melt, of thermoplastics mixed with polyacetal.
This object is achieved by means of a process for reducing formaldehyde emission from thermoplastics mixed with polyacetal, which comprises melting the thermoplastic mixture in the presence of at least one stabilizer, i.e. a chemical compound having a stabilizing effect, in a mixing assembly and subjecting it to further processing steps.
Surprisingly, it has been found that this process can drastically reduce formaldehyde emission.
This is all the more surprising since the processing conditions selected during the processing in the melt of thermoplastics other than polyacetal, and in particular during their recycling, are markedly less favorable than those for the processing of pure polyacetal.
It is also surprising since it is known that polyacetal can decompose when even small amounts of inappropriate foreign materials are present.
The present invention therefore provides a process for reducing formaldehyde emission from thermoplastic mixtures.
The invention further provides the use of stabilizers in the process of the invention, i.e. for reducing formaldehyde emission in thermoplastic mixtures.
The process of the invention is in principle suitable for the processing of any thermoplastic mixtures which comprise at least one thermoplastic other than polyacetal but contaminated with polyacetal.
Examples of thermoplastics are described in Saechtling, Kunststoff-Taschenbuch [Plastics Handbook], Hanser Verlag, 1998, 27th Edition, on pp. 375-534, incorporated herein by way of reference.
For the purpose of the invention, the term thermoplastic polymers in principle includes all of the known synthetic, natural or, modified natural polymers which can be processed in the melt, with the exception of polyacetal.